The present invention relates to a method of fabricating a nitride semiconductor device such as a semiconductor laser diode expected to be applied to the fields of optical information processing and the like.
Recently, a nitride semiconductor of a group III-V compound, that is, a compound including nitride (N) as a group V element, is regarded as a promising material for a short-wavelength light emitting device due to its large energy gap. In particular, a gallium nitride-based compound semiconductor (AlxGayInzN, wherein 0xe2x89xa6x, y, zxe2x89xa61 and x+y+z=1) has been earnestly studied and developed, resulting in realizing a practical blue or green light emitting diode (LED) device. Furthermore, in accordance with capacity increase of an optical disk unit, a semiconductor laser diode lasing at a wavelength of approximately 400 nm is earnestly desired, and a semiconductor laser diode using a gallium nitride-based semiconductor is to be practically used.
Now, a conventional gallium nitride-based semiconductor laser diode will be described with reference to a drawing.
FIG. 11 shows the sectional structure of the conventional gallium nitride-based semiconductor laser diode showing laser action. As is shown in FIG. 11, the conventional semiconductor laser diode includes a buffer layer 302 of gallium nitride (GaN), an n-type contact layer 303 of n-type GaN, a first cladding layer 304 of n-type aluminum gallium nitride (AlGaN), a first light guiding layer 305 of n-type GaN, a multiple quantum well (MQW) active layer 306 including gallium indium nitride layers having different composition ratios of indium (Ga1xe2x88x92xInxN/Ga1xe2x88x92yInyN, wherein 0 less than y less than x less than 1), a second light guiding layer 307 of P-type GaN, a second cladding layer 308 of p-type AlGaN and a p-type contact layer 309 of p-type GaN successively formed on a substrate 301 of sapphire by, for example, metal organic vapor phase epitaxial growth (MOVPE).
An upper portion of the second cladding layer 308 and the p-type contact layer 309 are formed into a ridge with a width of approximately 3 through 10 xcexcm. A lamination body including the MQW active layer 306 formed on the semiconductor substrate 301 is etched so as to expose part of the n-type contact layer 303, and the top face and the side faces of the etched lamination body are covered with an insulating film 310. In a portion of the insulating, film 310 above the p-type contact layer 309, a stripe-shaped opening is formed, and a p-side electrode 311 in ohmic contact with the p-type contact layer 309 through the opening is formed over a portion of the insulating film 310 above the ridge. Also, on a portion of the n-type contact layer 303 not covered with the insulating film 310, an n-side electrode 312 in ohmic contact with the n-type contact layer 303 is formed.
In the semiconductor laser diode having the aforementioned structure, when a predetermined voltage is applied to the p-side electrode 311 with the n-side electrode 312 grounded, optical gain is generated within the MQW active layer 306, so as to show laser action at a wavelength of approximately 400 nm. The wavelength of the laser action depends upon the composition ratios x and y or the thicknesses of the Ga1xe2x88x92xInxN and Ga1xe2x88x92yInyN layers included in the MQW active layer 306. At present, the semiconductor laser diode having this structure has been developed to show continuous laser action at room temperature or more.
It is generally well known that the growth temperature for growing a nitride semiconductor crystal by the MOVPE is changed in accordance with the composition ratio of a group III element introduced into gallium nitride (GaN).
It is reported that, in growing a semiconductor of, for example, gallium indium nitride (GaInN), nitrogen (N2) is preferably used as a material carrier gas with the growth temperature for the semiconductor set to approximately 800xc2x0 C. (Applied Physics Letters, Vol. 59, pp. 2251-2253, 1991).
On the other hand, it is also known that the first and second cladding layers 304 and 308 and the first and second light guiding layer 305 and 307 not including indium are preferably grown at a growth temperature of 1000xc2x0 C. or more with hydrogen (H2) used as a carrier gas.
The fabrication processes for these semiconductor layers are disclosed in, for example, Japanese Laid-Open Patent Publication No. 6-196757 or 6-177423.
The outline of the processes will now be described with reference to FIG. 11.
First, with hydrogen introduced onto a substrate 301, the principal plane of the substrate 301 is subjected to a heat treatment at a temperature of approximately 1050xc2x0 C. Then, after lowering the substrate temperature to approximately 510xc2x0 C., ammonia (NH3) and trimethylgallium (TMG), that is, mutually reactive gases, are introduced onto the substrate 301, so as to grow a buffer layer 302. Thereafter, with the introduction of TMG stopped, the substrate temperature is increased to approximately 1030xc2x0 C., and TMG and monosilane (SiH4) are introduced onto the substrate 301 with hydrogen used as a carrier gas, thereby successively growing an n-type contact layer 303, a first cladding layer 304 and a first light guiding layer 305, whereas trimethylaluminum (TMA) is additionally introduced as a group III material gas in growing the first cladding layer 304.
Next, the introduction of the material gases is stopped, the substrate temperature is lowered to approximately 800xc2x0 C., and the carrier gas is changed to nitrogen. Subsequently, trimethylindium (TMI) and TMG are introduced onto the substrate 301 as the group III material gases, thereby growing a MQW active layer 306.
Then, the introduction of the group III material gases is stopped, the substrate temperature is increased to approximately 1020xc2x0 C., and a group III material gas, that is, TMG and TMA if necessary, and cyclopentadienylmagnesium (Cp2Mg) including a p-type dopant are introduced onto the substrate 301, thereby successively growing a second light guiding layer 307, a second cladding layer 308 and a p-type contact layer 309.
After growing the MQW active layer 306, as a protection film for the active layer in increasing the temperature from 800xc2x0 C. to 1020xc2x0 C., a semiconductor layer of GaN is formed according to the description of Japanese Laid-Open Patent Publication No. 9-186363 or a semiconductor layer of Al0.2Ga0.8N is formed according to description of, for example, Japanese Journal of Applied Physics (Vol. 35, pp. L74-L76, 1996).
In general, the vapor phase epitaxial growth is conducted in an atmosphere of reduced pressure lower than the atmospheric pressure, the atmospheric pressure or increased pressure lower than approximately 1.5 atm.
A technique to suppress defects from occurring on an interface between a substrate and gallium nitride by growing gallium nitride on a substrate of sapphire by selective growth or the like is recently tried. It is reported with respect to this technique that gallium nitride with a flat face and high crystal quality can be obtained by conducting the vapor phase epitaxial growth under reduced pressure in particular.
As described so far, as a characteristic of growth of a gallium nitride-based semiconductor, different carrier gases are used in growing a layer including indium, namely, the MQW active layer 306, and layers not including indium, such as the first cladding layer 304 and the first light guiding layer 305. In general, nitrogen is used for growing the former layer and hydrogen is used for growing the latter layers.
Accordingly, in the fabrication of a semiconductor laser diode, particularly in forming a multilayer structure including double heterojunction layers sandwiching an active layer by the vapor phase epitaxial growth, it is necessary to change the carrier gas before and after forming the active layer. Also, the substrate temperature is changed at the same time. In changing the carrier gas, the introduction of the group III material gases such as TMG is stopped, and the substrate is placed in an equilibrium state where no crystal grows.
However, in the aforementioned conventional method of fabricating a nitride semiconductor device, the crystal face of the grown semiconductor layer is exposed to a high temperature of approximately 1000xc2x0 C. and reduced pressure lower than 1 atm while the substrate is placed in the equilibrium state where the introduction of the group III material gases is stopped. As a result, there arises a problem that constituent elements are released (re-evaporated) from the crystal face.
In particular, quality degradation of the first cladding layer 304 and the first light guiding layer 305 formed below the MQW active layer 306, particularly the first cladding layer 30 including 10% of aluminum in the aforementioned publication, leads to quality degradation of the MQW active layer 306. This degradation results in lowering the luminous efficiency and degrading operation characteristics, for example, increasing a threshold current, of the resultant light emitting diode or semiconductor laser diode.
Furthermore, it is recently reported in Journal of Electronic Materials (Vol. 28, No. 3, pp.287-289, 1999) that when gallium nitride is grown under increased pressure, an etch pit density can be reduced so as to suppress point defects.
On the other hand, the present inventors have found the following problem: When a nitride semiconductor is simply grown under increased pressure exceeding the atmospheric pressure in the above-described equilibrium state, the concentration of material gases is so increased that vapor phase reactions of ammonia with trimethylaluminum and cyclopentadienylmagnesium are caused, resulting in producing intermediate reaction products through these intermediate reactions.
Accordingly, the material gases cannot be efficiently supplied onto the growth face of a crystal on the substrate, resulting in extremely lowering the growth rate or preventing magnesium (Mg), that is, the p-type dopant, from being introduced into the crystal.
Furthermore, when the flow rate of a carrier gas for carrying the material gases is increased for avoiding the production of the intermediate reaction product, the amount of gases flowing through a reaction tube is so large that vortexes and convections are caused in the air flow within the reaction tube. As a result, the crystal cannot be grown under stable conditions.
In consideration of the aforementioned conventional problems, an object of the invention is improving the crystal quality of a nitride semiconductor, particularly the crystal quality of an active region and its vicinity of a semiconductor light emitting device, so as to improve the operation characteristics such as luminous efficiency.
In order to achieve the object, according to the present invention, the growth ambient pressure is changed in growing nitride semiconductors in accordance with the composition ratio of a group III element included in each nitride semiconductor.
Specifically, in fabrication of a nitride semiconductor device, increased pressure is employed in growing a semiconductor layer including an element tending to re-evaporate, such as indium, and reduced pressure is employed in growing a semiconductor layer including an element tending to generate an intermediate reaction product, such as aluminum or magnesium.
Furthermore, when epitaxial lateral overgrowth (ELO) is used, reduced pressure is employed in growing a semiconductor layer directly from a seed crystal, and the growth pressure is appropriately set in accordance with the composition ratio of a group III element in growing another semiconductor layer.
Specifically, the first method of fabricating a nitride semiconductor device of this invention comprises plural steps of respectively growing plural nitride semiconductor layers on a substrate; and between a step of growing one nitride semiconductor layer and a step of growing another nitride semiconductor layer adjacent to the one nitride semiconductor layer among the plural steps, a step of changing a growth ambient pressure from a first growth ambient pressure to a second growth ambient pressure different from the first growth ambient pressure.
In the first method of fabricating a nitride semiconductor device, optimal growth ambient pressures can be set in accordance with compositions of the plural stacked nitride semiconductor layers. Therefore, crystal dislocations can be reduced in the nitride semiconductor layers to be grown, the nitride semiconductor layers can be efficiently doped, and a semiconductor crystal of an active layer in particular can be improved in the quality. As a result, the operation characteristics of the semiconductor device can be improved.
In the first method of fabricating a nitride semiconductor device, the first growth ambient pressure or the second growth ambient pressure is preferably a pressure lower than the atmospheric pressure. In this manner, in growing a nitride semiconductor layer including an element tending to produce an intermediate reaction product, the intermediate reaction in a vapor phase between materials can be suppressed without increasing the flow rates of material gases and carrier gases, resulting in stabilizing the growth of the crystal and improving the growth efficiency. Accordingly, the crystal quality can be improved.
In the first method of fabricating a nitride semiconductor device, among the plural nitride semiconductor layers, a nitride semiconductor layer grown under the pressure lower than the atmospheric pressure preferably includes aluminum or magnesium. In general, since a nitride semiconductor layer including aluminum has a larger energy gap and a smaller refractive index than an active layer, such a nitride semiconductor layer is used as a cladding layer for sandwiching the active layer. Accordingly, the crystal quality of semiconductor layers formed in the vicinity of the active layer can be improved so as to improve the crystal quality of the active layer in this invention, and hence, the resultant semiconductor device can attain good operation characteristics. Also, since a nitride semiconductor layer including magnesium generally exhibits a p-type conductivity, a p-type semiconductor layer with good crystallinity can be efficiently obtained.
In the first method of fabricating a nitride semiconductor device, one of the first growth ambient pressure and the second growth ambient pressure is preferably a pressure higher than the atmospheric pressure and the other is a pressure lower than the atmospheric pressure. In this manner, a nitride semiconductor layer grown under the pressure lower than the atmospheric pressure can be improved in its growth efficiency because the production of an intermediate reaction product can be suppressed as described above. In addition, a nitride semiconductor layer grown under the pressure higher than the atmospheric pressure can be improved in its crystal quality even when it includes an element tending to re-evaporate.
In this case, among the plural nitride semiconductor layers, a nitride semiconductor layer grown under the pressure higher than the atmospheric pressure preferably includes indium. Since indium nitride has such a high vapor pressure that nitrogen can be easily re-evaporated during the growth, when the nitride semiconductor layer including indium is thus grown under increased pressure, the re-evaporation of nitrogen can be suppressed.
Also in this case, the nitride semiconductor layer including indium is preferably an active layer. In general, an active layer of a double heterojunction type of a light emitting device is required to have the smallest energy gap and the largest refractive index, and hence, a nitride semiconductor including indium is used as the active layer. Accordingly, the crystal quality of the active layer can be definitely improved in this invention.
In the first method of fabricating a nitride semiconductor device, the step of growing the one nitride semiconductor layer and the step of growing the adjacent nitride semiconductor layer are preferably conducted at different growth temperatures. In general, a nitride semiconductor mainly including gallium is grown at a growth temperature exceeding 1000xc2x0 C. However, when the nitride semiconductor layer includes an element such as indium having a high vapor pressure during the growth, the re-evaporation of nitrogen from indium nitride can be suppressed by setting the growth temperature to a lower temperature. Thus, the crystal quality can be definitely improved.
The second method of fabricating a nitride semiconductor device of this invention comprises the steps of forming plural seed crystals on a substrate; selectively growing, on the substrate, a first nitride semiconductor layer from the plural seed crystals under a first growth ambient pressure; and growing, on the first nitride semiconductor layer, a second nitride semiconductor layer under a second growth ambient pressure different from the first growth ambient pressure.
According to the second method of fabricating a nitride semiconductor device, in the first nitride semiconductor layer grown from the plural seed crystals in the lateral direction (the direction along the substrate surface), the lateral growth can be accelerated when it is grown under the first growth ambient pressure of, for example, reduced pressure. Therefore, the nitride semiconductor layer with a flat face can be formed over the substrate. Furthermore, when the second nitride semiconductor layer grown on the first nitride semiconductor layer is grown under the second ambient pressure different from the first ambient pressure, for example, under an optimal ambient pressure as in the first method of fabricating a nitride semiconductor device, the nitride semiconductor device with good quality can be formed on the first nitride semiconductor layer including few defects.
In the second method of fabricating a nitride semiconductor device, the first growth ambient pressure is preferably lower than the atmospheric pressure.
In the second method of fabricating a nitride semiconductor device, a first growth temperature employed for growing the first nitride semiconductor layer and a second growth temperature employed for growing the second nitride semiconductor layer are preferably different from each other. In this manner, the crystal growth of the first nitride semiconductor layer and the second nitride semiconductor layer grown thereon can be individually optimized.
In this case, the second growth temperature is preferably higher than the first growth temperature. In this manner, the crystal orientation of the second nitride semiconductor layer can be further improved.
In the second method of fabricating a nitride semiconductor device, the first nitride semiconductor layer preferably includes aluminum. In this manner, the surface of the first nitride semiconductor layer can be rigid, so as to prevent the surface of the first nitride semiconductor layer from degrading before starting growing the second nitride semiconductor layer. As a result, the crystallinity of the second nitride semiconductor layer can be improved.